It is a common practice to carry out, from a console, status control (start, stop and reset) and status indication (error status indication and run/stop indication) of a central processing unit, data reference and data writing (hereinafter termed "read/write") of a group of registers, etc.
In a random logic-controlled CPU, all the instructions from the console are assembled in a hard wired logic control unit. In a microprogram-controlled CPU, the controls for the instructions from the console are to be carried out by the microprogram control as far as possible.
In the case of a microprogram control system, an appropriate link between the instructions from the console and the microprogram control is required. Where it is desired to make the size of the central processing unit as small as possible, it is favorable to reduce the amount of hardware for such a link to the smallest possible extent.
Regarding the reading and writing of the contents of a group of registers within the central processing unit, the following two methods have hitherto been known:
One is a method in which the reading and writing from or into the group of registers is controlled directly by the microprogram control.
According to this method, an address at which microprogram control data is stored is supplied from the console to the central processing unit. The central processing unit sets the address in a microprogram address register upon the condition that the usual microprogram control is stopped, and the microprogram control data is delivered from a microprogram memory on the basis of the address, whereby reading and writing operations for designated registers are carried out.
The other method consists in controlling the reading and writing operations for a group of registers independently of microprogram control.
According to this method, a register address is delivered from the console to the central processing unit, and the central processing unit carries out the reading and writing of the contents of the registers on the basis of the register address, upon the condition that the usual microprogram control is stopped.
In either method, a large amount of hardware is required for the central processing unit in order to read and write the contents of the registers within the central processing unit from the console.